The invention relates to a composite body comprising at least a first part of ceramic material which is connected to a second part of metallic material, of ceramic material or of a mixture of metallic and ceramic material by means of a connection material which contains besides Al.sub.2 O.sub.3 also one of the oxides BaO, SrO, Sc.sub.2 O.sub.3, Y.sub.2 O.sub.3 and at least one rare earth oxide and whose composition lies within the following limits, expressed in mol.%:
Al.sub.2 O.sub.3 : 35-70 PA0 BaO: 0-20 PA0 SrO: 0-20 PA0 Sc.sub.2 O.sub.3 : 2-40 PA0 Y.sub.2 O.sub.3 : 2-40 PA0 rare earth oxide 5-40.
A ceramic material is to be understood to mean in this description and the appended claims a crystalline oxidic material at least containing alumina. This may be, for example, monocrystalline sapphire. Another possibility is densely dintered polycrystalline alumina ot yttrium aluminum garnet. These materials, which are suitable as a wall material for discharge vessels of high-pressure discharge lamps, can be heated for a long time to about 1300.degree. C. and at such high temperatures have a high resistance to attack by alkali metals and halides. Heating for a long time to above 1300.degree. C. generally leads to a substantial evaporation of the relevant materials. Heating for a short time to 1750.degree. to 1800.degree. C. is possible, however, without disadvantageous consequences. Heating to above 1750.degree. to 1800.degree. C., even for a short time, leads to the aforementioned evaporation, however, to a great extent, and even to spontaneous formation of ruptures.
Rare earth oxides are to be understood to mean the following oxides: Gd.sub.2 O.sub.3, Tb.sub.2 O.sub.3, Dy.sub.2 O.sub.3, Ho.sub.2 O.sub.3, Er.sub.2 O.sub.3, Eu.sub.2 O.sub.3, Pr.sub.2 O.sub.3, Ce.sub.2 O.sub.3, Sm.sub.2 O.sub.3, Tm.sub.2 O.sub.3, Yb.sub.2 O.sub.3, Lu.sub.2 O.sub.3, La.sub.2 O.sub.3, Nd.sub.2 O.sub.3.
A composite body of the kind mentioned in the opening paragraph is known from European Patent Application No. 0060582A1. The known connection materials preferably contains BaO or SrO as a constituent inhibiting the crystal growth in a quantity of at most 20 mol.%. Such a composite body may be a high-pressure discharge lamp provided with a discharge vessel having a ceramic wall through which a current supply member is passed, which is connected thereto in a gas-tight manner by means of the connection material. Frequently used high-pressure discharge lamps are high-pressure sodium lamps and high-pressure metal halide lamps. In both lamp types, the wall of the discharge vessel is loaded at a high temperature in the operating condition and the filling of the discharge vessel is partly ionized. The connection material used in such lamps therefore should be resistant to attack by sodium and metal halides at high temperatures.
A gas-tight connection providing a high resistance to the attack by sodium and halides can be obtained by means of the known connection material. In general, a gas-tight connection is obtained by a fine crystallization of the connection material. It should be noted that in general the thermal expansion coefficients of the interconnected parts are different so that stresses may be produced in the connection material. In the case of a fine crystallization, the possibility of crack formation is smaller than in the case of coarse crystalline connection material.
A further advantage of a fine crystalline connection material is that during the crystallization process the possibility of occlusions and micro-cracks is considerably smaller than in coarse crystallized connection material.
The known connection material satisfies to a great extent the requirement of fine crystallization, inter alia due to the preferably added constituents BaO and SrO, respectively, which have an inhibiting effect on the crystal growth. However, it has been found that the known connection material in the connection with an alumina-containing ceramic part of the composite body may lead to formation of .beta.-aluminate structures. A .beta.-aluminate crystal structure is highly sensitive to attack by especially sodium and so very undesirable in the case of a high-pressure discharge lamp with sodium as filling constituent.